Polytetrafluoroethylene molding powder

ABSTRACT

Polytetrafluoroethylene molding powder which comprises 0.01 to 1% by weight of a perfluoro(vinyl ether) unit and which has a heat of crystallization of 18.0 to 25.0 J/g. The molding powder provides a molded article which has a flex life of not less than 7 million cycles, a creep resistance (total deformation at 200° C.) of not more than 20%, and a creep resistance (total deformation at 25° C.) of not more than 15%. The polytetrafluoroethylene molding powder which is excellent in both creep resistance and flex fatigue resistance is provided as a molding material for parts which are repeatedly bent such as pump.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation application of International Application No.PCT/JP93/00140, filed Feb. 4, 1993, which entered the U.S. nationalphase and was assigned U.S. Ser. No. 08/129,061, now abandoned filed onNov. 15, 1993.

TECHNICAL FILED

The present invention relates to a polytetrafluoroethylene (which may behereinafter referred to as "PTFE") molding powder which is used toobtain a molded article having an improved flex fatigue resistance, aprocess for preparing the same and a pelletized powder made of the same.

BACKGROUND ART

A suspension-polymerized polymer of tetrafluoroethylene (TFE) isexcellent particularly in chemical resistance and heat resistance, andis used as a molding material for a variety of molded articles, thoughthe suspension-polymerized polymer is not melt-processable. Among suchmolded articles, articles which are bent repeatedly such as pump,bellows and diaphragm are required to have simultaneaously flex fatigueresistance (flex life) and creep resistance.

A molded article made of homopolymer powder of TFE has an inferior creepresistance though the article has a sufficiently long flex life (e.g.with regard to M-12 made by Daikin Industries, Ltd., a flex life is 7.2million cycles and a creep resistance (200° C., load: 70 kgf/cm², totaldeformation) is 26.7%). Therefore, the homopolymer powder of TFE is nota fully satisfactory polymer as materials for molded articles which arerequired to have a high strength under a high load.

Known as one method to improve this creep resistance, is to addinorganic and organic various filling materials (fillers), namely, glassfiber, carbon powder, graphite, molybdenum disuifide, bronze, polymide,polyamideimide, polyphenylene oxide, polyallylene sulfide and the like(JP-A-24252/1977). Though the addition of the fillers improves the creepresistance, problems such as coloring and discoloration occur in moldingprocess because of impurities derived from the fillers, or problems suchas increase of osmosis or permeation of liquid or gas through the moldedarticle because of the formation of voids due to the addition offillers.

To improve these problems, there are disclosed methods in which variousadditives are added or the fIlers are surface-treated inJP-B-18696/1985, JP-B-21178/1985, JP-B-57093/1990 and the like. However,such methods cannot solve completely the problems due to the fillers,and increase the number of process steps and costs for production.

Known as the other method to improve t he creep resistance, is tocopolymerize TFE with a modifier such as perfuluoro(alkyl vinyl ether),or perfluoro(alkoxyalkyl vinyl ether) (JP-B-4 6794/1976,JP-B-31524/1984). These methods try to improve the creep resistance withkeeping melt-unprocessability by copolymerization with the reformingagent. JP-B-46794/1976 discloses a preparation ofpolytetrafluoroethylene by copolymerization with perfluoro(alkyl vinylether) in 0.02 to 0.26% by weight. Actually, the copolymerization iscarried out at 65° C. in the case of perfluoro(propyl vinyl ether), andthe product has a melt viscosity of 1×10⁹ poise and a standard specificgravity of 2.175 to 2.186. Also, in the case of perfluoro(methyl vinylether), the copolymerization is carried out at a high copolymerizationtemperature of 65° C. and a high initiator concentration of 100 to 225ppm, and the resulting polytetrafluoroethylene has a lower molecularweight than the homopolymer.

This polytetrafluoroethylene has a problem that the flex fatigueresistance (flex life) of the molded article is lowered, while the creepresistance is improved in comparison with the homopolymer of TFE.

As products of such a polytetrafluoroethylene, for example, TFM-1700(Farbwerke Hoechst Aktien Gesellschaft) and TG-70J (Du pont-MitsuiFluorochemicals Co., Ltd.) are commercially available. When measuringthe flex fatigue resistance (flex life), the creep resistance and thelike of the molded articles of these products, though the creepresistance is improved, the flex fatigue resistance is lowered incomparison with the homopolymer of polytetrafluoroethylene.

Further, JP-B-31524/1975 discloses copolymerization of TFE withperfluoro(alkyl vinyl et her) in 0.0004 to 0.00 29% by mole. However,the content of perfluoro(alkyl vinyl ether) is not sufficient to improvethe creep resistance.

With respect to relation of the flex ilfe and the creep resistance ofpolytetrafluoroethylene molded article, there is a general tendency thatthe creep resistance decreases when the flex life is improved and theflex life is shortened when the creep resistance is improved. However,the inventors have found the specific characteristics that the decreaseof the creep resistance can be controlled even if the flex life isimproved, when a perfluoro(alkyl vinyl ether) content and a heat ofcrystallization are within the particular ranges, and have completed thepresent invention.

It is an object of the present invention to provide a molding powder forobtaining a polytetrafluoroethylene molded article which has both thecreep resistance and the flex fatigue resistance (flex life)simultaneously.

DISCLOSURE OF THE INVENTION

The polytetrafluoroethylene molding powder according to the presentinvention is a polytetrafluoroethylene molding powder which is notmelt-processable and which has a specific surface area of 0.5 to 0.9 m²/g and an average particle size of not more than 100 μm, characterizedin that the polytetrafluoroethylene comprises 0.01 to 1% by weight,preferably 0.03 to 0.20% by weight of a perfluoro(vinyl ether) unithaving the general formula (I): ##STR1## wherein X is a perfluoroalkylgroup having 1 to 6 carbon atoms or a perfluoroalkoxyalkyl group having4 to 9 carbon atoms, and has a heat of crystallization of 18.0 to 25.0J/g, preferably 18.0 to 23.5 J/g measured by a differential scanningcalorimeter, and that a molded article made of the powder has

(a) a flex life of not less than 7 million cycles,

(b) a creep resistance (total deformation) of not more than 20% at 200°C., and

(c) a creep resistance (total deformation) of not more than 15% at 25°C.

The molding powder is obtained by suspension polymerization oftetrafluroethylene and a perfluoro(vinyl ether) having the generalformula (II):

    CF.sub.2 ═CF--O--X                                     (II)

wherein X is as defined above, at a temperature of 40° to 55° C. byusing a persulfate as an initiator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanation view showing a method to find a heat ofcrystallization from a DSC chart of a differential scanning calorimeteradopted in the present invention; and

FIG. 2 is a schematic partially cut away longitudinal sectional view ofa molded article produced in Examples 19 to 22 and Comparative Examples22 to 24.

BEST MODE FOR CARRYING OUT THE INVENTION

Examples of the perfluoro(vinyl ether) (II) used are a perfluoro(alkylvinyl ether) such as perfluoro(methyl vinyl ether), perfluoro(ethylvinyl ether), perfluoro(propyl vinyl ether), perfluoro(butyl vinylether), perfluoro(pentyl vinyl ether) or perfluoro(hexyl vinyl ether); aperfluoro(alkoxyalkyl vinyl ether) such as perfluoro(2-methoxypropylvinyl ether) or perfluoro(2-propoxypropyl vinyl ether) and the like. Acontent of these perfluoro(vinyl ether) units in PTFE is 0.01 to 1% byweight, preferably 0.03 to 0.20% by weight. When the content is too low,the creep resistance decreases. When the content is too high, thetensile strength and the crack resistance decrease, and the improvementof creep resistance is not so enhanced despite of using a large amountof expensive perfluoro vinyl ethers, resulting in economicaldisadvantages.

Suspension polymerization is carried out under polymerization.conditions which suffice the above-mentioned characteristics. It is veryimportant to adequately regulate a polymerization temperature, and akind of initiator and an initiator concentration.

Generally, suspension polymerization of tetrafluoroethylene withperfluoro(vinyl ether) has been carried out at a polymerizationtemperature of 0° to 100° C. by using an organic or inorganic peroxidetype initiator or a redox type initiator at a relatively highconcentration of the initiator. Both of such conditions are required tobe lowered for obtaining polytetrafluoroethylene having a high molecularweight. However, the regulation and combination of those conditions areunexpectedly difficult in industrial scale.

In the present invention, polymerization conditions are adopted in whicha persulfate preferably having a half life between 18 to 120 hours at55° C. is used as an initiator, a polymerization temperature iscontrolled at 40° to 55° C. and, more preferably, a charge amount of theinitiator is such an amount that a decomposition amount of the initiatorduring three hours from the beginning of the polymerization is 4×10⁻⁷ to8×10⁻⁶ mole/liter in terms of a concentration on the basis ofpolymerization water. When the half life of the initiator is shorterthan 18 hours or the decomposition amount of the initiator during threehours from the beginning of the polymerization is greater than 8×10⁻⁶mole/liter, the molecular weight does not increase, and when the halflife is longer than 120 hours or the decomposition amount is smallerthan 4×10⁻⁷ mole/liter, the condition are not proper for industrialscale, since the polymerization rate is too slow.

Examples of the persulfate are ammonium persulfate (APS), patassiumpersulface (KPS) and the like.

In the polymerization, it is preferable to dissolve a buffer such asammonium carbonate in the polymerization water for keeping a pH of thepolymerization water within a basic pH range to prevent a polymerizationvessel from corrosion as little as possible.

Further, when occasion demands, the polymerization may be carried out byadding a telogenically inactive dispersing agent such as a salt of aperfluorocarboxylic acid, for example, ammonium perfluorooctanoate,ammonium perfluorononanoate or the like in an amount of 1 to 200 ppm onthe basis of the polymerization water. As a result of the polymerizationwith a small amount of the dispersing agent is added in this way, aspecific surface area of the resulting powder is enlarged (5.0 to 9.0 m²/g), and a pressure transferability is improved in the molding. Further,molded articles made of this powder is well dense and also has excellentelectric properties.

The polymerization time is about 8 to 25 hours. The resultingpolytetrafluoroethylene has a high molecular weight and has a heat ofcrystallization of 18.0 to 25.0 J/g, preferably 18.0 to 23.5 J/gmeasured by a differential scanning calorimeter described hereinafter.The resulting polytetrafluoroethylene has usually a melt viscosity (380°C.) of about 5.0×10⁹ to 1×10¹¹ poises, preferably 5.0×10⁹ to 7.0×10¹⁰poises.

The polytetrafluoroethylene raw powder obtained by the suspensionpolymerization is dried and ground in a usual way to get a moldingpowder having a specific surface area of 0.5 to 9.0 m² /g and an averageparticle size of not more than 100 μm, preferably not more than 50 μm.This molding powder can be processed as it is by a usual moding methodfor polytetrafluoroethylene to provide molded articles having theabove-mentioned performances. As the molding method, there may beemployed usual methods such as compression molding, ram extrusionmolding and isostatic molding.

The resulting molded article has

(a) a heat of crystallization which is approximately the same as that ofthe molding powder, measured by a differential scanning calorimeter,

(b) a flex life of not less than 7 million cycles, preferably not lessthan 10 million cycles,

(c) a creep resistance (total deformation) at 200° C. of not more than.20%, and

(d) a creep resistance (total deformation) at 25% of not more than 15%.

Further, the resulting molded article has a feature that SVI (stretchingvoid index) is low. SVI is a physical property described in Item 10.9 ofASTM D4895-89 which is an index of the difference between a specificgravity of the molded article before tensile test and a specific gravityof the molded article after tensile test. This is regarded as anumerical value which represent an amount of the voids produced bystretching the molded article. That is to say, smaller the SVI value is,the void is more difficult to be formed under tension. The moldedarticle made of the polytetrafluoroethylene powder according to thepresent invention has a SVI value of not more than 50, preferably notmore than 40.

The polytetrafluoroethylene molded article having such performances hasnot yet been known until now, and can be provided by the presentinvention for the first time.

The polytetrafluoroethylene molding powder of the present invention mayalso be pelletized by known agglomeration methods. The agglomeration canbe carried out, for example, by homogeneously mixing with stirring themolding powder in a two phase liquid medium comprising water and anorganic liquid having a surface tension of not more than about 40dyne/cm at 25° C. Examples of the organic liquid are, an aliphatichydrocarbon such as pentane or dodecane; an aromatic hydrocarbon such asbenzene, toluene or xylene; a halogenated hydrocarbon such astetrachloroethylene, trichloroethylene, chloroform, chlorobenzene,trichlorotrifluoroethane, monofluoro trichloromethane ordifluorotetrachloroethane; and the like. Among them, the halogenatedhydrocarbon is preferable, and a chlorinated hydrocarbon or afluorinated hydrocarbons each as 1,1,1-trichloroethane,1,1-dichloro-2,2,3,3,3-pentafluoropropane,1,3-dichloro-1,1,2,2,3-pentafluoropropane,1,1-dichloro-2,2,2-trifluoroethane or 1,1-dichloro-1-fluoroethane isparticularly preferable, because they are nonflammable and suffice arequirement of such a Ozone problem or the like. These organic liquidsmay be used alone or in combination of two or more.

An apparent density of the resulting pelletized powder is 2 to 3 timeshigher than that of the primary molding powder before the agglomeration,and the pelletized powder is excellent in flowability and handling.Therefore, the pelletized powder is adequate also for automaticcompression molding. More specifically, the pelletized powder has

(a) an average particle size of 150 to 1000 μm,

(b) an apparent density of 0.5 to 1.0 g/cm³, and

(c) a particle size distribution in which not less than 30% by weight,preferably 35 to 45% by weight of the powder has a particle size 0.7 to1.3 times larger than the average particle size. Within the range of 35to 45% by weight, the apparent density tends to be higher than the othercases.

Further, the molded article made of the pelletized powder has

(d) a flex life of not less than 5 million cycles, and

(e) a SVI of not more than 50.

The pelletized powder according to the present invention is alsoexcellent in welding and electric insulation. It is known that apolytetrafluoroethylene powder comprising a perfluoro(vinyl ether) unithas an excellent welding and electric insulation. The welding and theelectric insulation can be measured by using the evaluation methodsdescribed in JP-B-39 105/1991 described hereinafter.

Further, the polytetrafluoroethylene molding powder of the presentinvention may be optionally admixed with a filler. The molding powderadmixed with the filler may be pelletized in a known agglomerationmethod.

The filler for admixing is not particularly restricted, and an admixingamount of the filler is such that a ratio by weight ofpolytetrafluoroethylene (PTFE)/filler is 20 to 99/80 to 1, preferably 30to 99/70 to 1.

Specifically, non-restricted examples are, for instance, glass fiber (3to 30% by weight: admixing amount, hereinafter the same), graphitepowder (3 to 30% by weight), bronze powder (10 to 80% by weight ), goldpowder (10 to 80% by weight), silver powder (10 to 80% by weight),copper powder (10 to 80% by weight), stainless steel powder (3 to 50% byweight), stainless steel fiber (3 to 50% by weight), nickel powder (3 to50 % by weight), nickel fiber (3 to 50% by weight), molybdenum disulfidepowder (3 to 30% by weight), cokes powder (5 to 30% by weight), carbonfiber (3 to 30% by weight), aromatic heat resistant resin powder such aspolyoxybenzoylpolyester (5 to 30% by weight), boron nitride powder (1 to20% by weight), polyimide powder (5 to 30% by weight),tetrafluoroethyleneperfluoro(alkyl vinyl ether) copolymer (PFA) powder(5 to 30% by weight), fluorinated mica powder (5 to 40% by weight),carbon black (1 to 30% by weight), polyphenylene sulfide powder (1 to30% by weight), mixture thereof, and the like. Further, the filler maybe surface-treated with a PTFE dispersion or a silane coupling agent.When two or more fillers are used, preferable combination is glass fiberand graphite powder, glass fiber and molybdenum disulfide powder, bronzepowder and molybdenum disulfide powder, bronze powder and carbon fiber,graphite powder and cokes powder, graphite powder and aromatic heatresistant resin powder, carbon fiber and aromatic heat resistant resinpowder, or the like. The mixing method may be either wet method or drymethod.

The polytetrafluoroethylene molding powder of the present invention maybe admixed with usual additives, in addition to other than theabove-mentioned filler, for example, a colorant, an antistatic agent,and the like.

Because of its excellent characteristics, the molding powder accordingto the present invention is useful as it is or after blending with thefiller, as a raw material for molded articles for the following uses.For example, molded articles which need a flex fatigue resistance suchas bellows, diaphragm, hose, piston ring and butterfly valve; moldedarticles which need a creep resistance such as ball valve sheet,packing, gasket, piston ring, bellows, disphragm and butterfly valve;molded articles which need a gas permeation resistance such as bellows,diaphragm, hose, packing and gasket; and the like.

Among them, the molding powder of the present invention is particularlyadequate for a bellows and a diaphragm of chemical pump which needschemical resistance, flex fatigue resistance, creep resistance and gaspermeation resistance. The chemical pump is used in chemical industryand semiconductor manufacturing apparatus to transfer strongly corrosivefluids, for example, gases such as fluorine, hydrogen chloride, sulfuroxide, nitrogen oxide and phosgene; a liquid such as hydrogen fluoride,hydrochloric acid, sulfuric acid, nitric acid, phosphorus oxychloride,thionyl chloride, sulfuryl chloride, chromic acid, other various organicacids and acid halides. Until now, polytetrafluoroethylene is used as amaterial for molding for a bellows and a diaphragm which are flexibleand movable parts of the above-mentioned chemical pump mainly due to achemical resistance of polytetrafluoroethylene (JP-A-32422/1972,JP-A-2320/1973, JP-A-151465/1985, JP-A-116303/1 .989). However, all ofthese prior arts relate to improvements of structures of apparatusesinto which a bellows and a diaphargm are incorporated, and do not relateto an improvement of the flex fatigue resistance and the creepresistance. An example for improving the creep resistance is disclosedin JP-A-127976/1986 in which a melt-processable perfluoro(alkyl vinylether) copolymer is mixed in polytetrafluoroethylene. However, the flexfatigue resistance is not improved in this example.

As mentioned above, the molded article of the present invention isexcellent in both creep resitance and flex fatigue resistance, and alsoin gas permeation resistance. Therefore, a chemical pump comprising abellows and a diaphragm of the present invention is an excellent pumpwhich is maintenance-free and can be used for a long time.

The molding powder mixed with the filler is particularly preferably usedfor molded articles which need a creep resistance at a high temperatureand an abrasion resistance. However, the use is not limited to them.Specifically, examples are piston ring, sealing material for powersteering, various mechanical sealing materials, bearing, gasket, valvesheet, packing, bearing pad and the like. The molding powder mixed withthe filler is particularly suitable for sliding parts and sealingmaterials of automobiles. A method for molding them may be the same asthat for a conventional PTFE powder which is not melt-processable.Examples are (automatic) compression molding, ram extrusion molding,isobaric pressure molding and the like. Further, because a particularPTFE having a low heat of crystallization itself is excellent inabrasion resistance, physical properties (such as tensile strength,elongation and the like) of the molded article can also be improved bydecreasing an amount of the filler.

Measuring methods for the measured values described in the specificationare explained here inbelow.

(Measurement of perfluoro(vinyl ether) content)

The content is calculated from characteristic absorptions (between 1040cm⁻¹ to 890 cm⁻¹ in the case of perfluoro (propyl vinyl ether)) ininfrared spectroscopy.

(Measurement of average particle size)

An average particle size is measured during a vibration time of 10minutes according to JIS K 6891-5.4.

(Measurement of particle size distribution)

This is a proportion by weight of particles which have a diameter 0.7 to1.3 times larger than the average particle size to all particles. Theproportion is calculated by multiplying the average particle size by 0.7to 1.3 and plotting the calculated value into a cumulative curve.

(Measurement of specific surface area)

The specific surface area is measured by MONOSORB (available from YuasaIonics Kabushiki Kaisha) according to a nitrogen adsorption methoddescribed in Analytical Chemistry, vol. 30, page 1387 (1985).

(Measurement of melt viscosity)

A mold having an inside diameter of 50 mm is charged with 210 g of thepowder, the powder is gradually pressurized for about 30 seconds up to afinal pressure of about 300 kg/cm², and the final pressure is maintainedfor another 5 minutes to get a preform. The preform is is taken out ofthe mold, and the temperature of the preform is raised at a rate of 50°C./h up to 365° C. in an air oven, and then after the temperature ismaintained for 5.5 hours, the preform is cooled at a rate of 50° C./hdown to room temperature to get a cylindrical sintered article A. Thissintered article is skived along a side face to produce a 0.5 mm thicksheet B. Four to Five mm wide and 15 mm long test pieces are cut fromthis sheet and the width and the thickness are measured correctly tocalculate a cross section. Thermoflex TMA (Rigaku DenkiKabushiki Kaisha)is used. Clamping molds are attached on both ends of the test piece suchthat a distance between two clamping portion is 1 cm. This mold-testpiece assembly is put in the cylindrical furnance, heated at a rate of20° C./min from room temperature to 380° C., and after maintaining thistemperature for 5 minutes, a load of about 15 g is applied. Anelongation during 60 minutes from 60 minutes after the application ofthe load to 120 minutes after is measured from a curve showing a changeof the elongation with 1 laps of time. The proportion of the elongation(dL_(T) /dT) against time (60 minutes) is calculated. A melt viscosity(η) is defined in the following eqation.

    η=W=L.sub.T ×g/3×(dL.sub.T /dT)×A.sub.T

W: tension load (g)

L_(T) : length (cm) of the test piece at 380° C. (longer than that atroom temperature by about 8%)

A_(T) : cross section of the test piece at 380° C. (larger than that atroom temperature by about 37%)

g: gravitational constant (980 cm/sec²)

(Measurement of heat of crystallization by differential scanningcalorimeter (DSC))

About 3 mg of the unsintered powder is weighed precisely, put in thespecial aluminium pan, and measurement is carried out by DSC(DSC-50 ofShimadzu Corporation). First, the aluminium pan is heated to 250° C.under a N₂ atomosphere. After maintained at the temperature, and thenthe pan is heated to 380° C., at a rate of 10° C./min to sufficientlymelt the crystal. Then, the pan is cooled from 380° C. to 250° C. at arate of 10° C./min, and the heat of crystallization point is measured.As shown in FIG. 1, a tangential line is drawn from a point of 275° C.to another end of the peak. An area surrounded by the curve of the peakand the tangential line gives a value of the heat of crystallization.FIG. 1 corresponds to Example 3.

(Measurement of flex life)

A 6.5 mm wide and 14 cm long test piece is cut away from the sheet Bproduced in the Measurement of melt viscosity. The number of times ofdouble bending at break of the test piece by applying a tension of 1.2kg to the test piece is measured, by means of the MIT testing machinedescribed in ASTM D2176-63T.

(Measurement of creep resistance)

From the cylindrical sintered article A produced in Measurement of meltviscosity, a test piece having a diameter of 11.3 mm and a height of 10mm is cut away along a direction parallel to the pressurizing direction.To the test piece is applied a load of 70 kgf/cm² at 200° C. accordingto ASTM D621. Deformation after retention of 24 hours is defined astotal deformation. And, after releasing the load and allowing to standfor another 24 hours at 200° C., deformation as compared with a lengthof the original test piece is defined as permanent deformation. Creepresistance at 25° C. is measured in the same way as that at 200° C.except that a load is 140 kgf/cm².

(Measurement of welding factor)

Welding factor is measured according to the method disclosed inJP-B-39105/1991.

A mold having an inside diameter of 50 mm is charged with 185 g of thepelletized molding powder, and the powder is gradually compressed forabout 30 seconds to a final pressure of about 150 kg/cm², and then thefinal pressure is maintained for another 5 minutes to get a preformhaving a diameter of 50 mm and a length of 45 mm.

Each of the cylindrical articles is sintered in a glass tube of 53 mmdiameter without loading, as follows:

The sintered article is heated up from 20° to 380° C. at a linear rateof 45° C./h, this temperature being maintained for 4 hours, then cooledto 20° C. at a linear rate of 45° C./h. This sintering and coolingprocess is repeated. The welding material obtained in this way isrotated to be cut to give a test specimen having a diameter of 20 m inthe center (outside the region of clamping). In the same way, comparisonspecimens which are not welded (i.e. are compression molded from theoutset in the full length of 90 mm) are prepared. These test specimensare subjected to a tensile test in order to determine the tensilestrength at break, following the guidelines of DIN Standard 53,455 andworking at a drawing speed of 30 mm/min. The welding factor is thequotient of the tensile force at break of the welded sample, divided bythe tensile force at break of the non-welded sample.

(Pore count)

A mold having an inside diameter of 50 mm is charged with 210 g of thepelletized powder, the powder is gradually pressurized for about 30seconds up to a final pressure of about 350 kg/cm², and the finalpressure is maintained for another 5 minutes to get a preform. Thepreform is taken out of the mold, and the temperature of the preform israised at a rate of 45° C./h up to 380° C. in an air oven, and thenafter the temperature is maintained for 4 hours, the preform is cooledat a rate of 45° C./h down to room temperature to get a cylindricalsintered article. Then, a 0.2 m thick sheet is skived from this sinteredarticle. This skived sheet having a width of 50 mm and a thickness of0.2 mm is passed through between the electrodes to which are applied adirect voltage of 5000 volts. The sheet is fed successively at a rate of10 cm/sec by a roll, and the number of pores (electrically defects) per1 m² of the sheet is measured. The upper electrodes are projecting likea reed screen, to touch a whole surface of the sheet (the substantiallysame condition as in a voltage is applied to the whole surface of thesheet). The number of pores are recorded by a computer.

(Measurement of SVI value)

SVI is measured according to the method described in ASTM 04895-89-10.9.

A mold having a diameter of 76 mm is charged with 29 g of the powder,and a molding pressure of 70 kgf/cm² is applied thereto and the pressureis maintained for 2 minutes. Then, the molding pressure is raised up to140 kgf/cm², and the pressure is maintained for another 2 minutes. Theobtained preform is taken out of the mold, and put in an air-electricfurnance kept at 290° C. Then, the preform is heated up to 380° C. at arate of 120° C./h, and after keeping the temperature at 380° C. for 30minutes, the article is cooled down to 294° C. at a rate of 60° C./h.This temperature is maintained for 24 minutes, and the article is takenout of the furnance and air-cooled. From the molded article in a discform obtained in this way, a test piece for the tensile test is punchedout by using a micro dumbbell standardized by ASTM. Difference ofspecific gravities before and after the tensile test of this test pieceis measured, and SVI is found by the equation described later. Here, thetensile test is carried out at a distance between chucks of 12.7 mm andan tensile rate of 5 mm/min, and a sample for mesuring a specificgravity is selected from the samples which have been broken at anelongation of 500%.

    SVI=(specific gravity before tensile test-specific gravity after tensile test)×1000

(Measurement of abrasion resistance)

The composition for molding is compression-molded and sintered to get acylindrical sintered article C in the same way as that ofabove-mentioned Measurement of melt viscosity except that a moldingpressure is 500 kgf/cm-². This sintered article C is cut out to get asample (25.6 mm φ/20 mm φ/15 mm length) for Suzuki-Matsubarafriction-abrasion tester. Suzuki-Matsubara friction-abrasion tester(Orientic K Kabushiki Kaisha) is used to conduct friction-abrasion testunder conditions that a material to be contact is an aliuminium material(JIS 2024P), a load is 4 kgf/cm², a rate is 1 m/sec and a test time is65 hours, to know a friction coefficient and an abrasion coefficient.

The abrasion coefficient is calculated according to the followingequation:

    K=W/(p×v×t)

Here, K is an abrasion coefficient, W is an abrasion amount (mm), p is aload (kgf/cm²), v is a rate (km/sec), t is a time (sec).

(Measurement of apparent density)

Apparent density is measured according to JIS K 6891-5.3.

(Measurement of tensile strength and elongation)

From the sheet B produced in Meaurement of melt viscosity, a test pieceis punched out by using the dumbbell 3 standardized in JIS. The testpiece is stretched at a rate of 200 mm/min by using an autograph of atotal load of 500 kg according to JIS K6891-5.8, to measure a tensilestrength and an elongation at break.

The present invention is explained by means of the following Examples.The present invention is not limited to the Examples.

EXAMPLE 1

A solution of 3.3 g of ammonium carbonate in 54.8 liters of deinoizedwater is charged in an autoclave of 170 liters, and stirred (110 r.p.m.)with an anchor type stirrer. After deaeration, the autoclave is chargedwith tetrafluoroethylene so as to be a pressure of 0.5 kg/cm² (gagepressure). After repeating this procedure three times, 85 g ofperfluoro(propyl vinyl ether) is introduced under pressure oftetrafluoroethylene. After raising the temperature of reaction system to5 0° C., tetrafluoroethylene is introduced under pressure until aninternal pressure of the reaction system reaches 8 kg/cm². To thisreaction system, 0.2 liter of an aqueous solution of 715 mg of ammoniumpersulfate (half-life at 55° C. is 46.3 hours. Decomposition amount ofammonium persulfate during three hours from the inintiation ofpolymerization is 1.2×10⁻⁶ mole/liter of polymerization water at apolymerization temperature of 50° C.) is added to initiate thepolymerization . The polymerization is carried out with continuoslyintroducing tetrafluoroethylene under pressure such that the internalpressure of the reaction system is maintained at 8 kg/cm², and iscontinued until 22.5% by weight of tetrafluoroethylene on the basis ofthe weight of the aqueous medium is consumed, then, the monomers aredischarged. After cooling down to room temperature, the resultingpolytetrafluoroethylene raw powder is taken out, and roughly pulverized.This crude powder is dried, and finely ground in a pulverizer to anaverage particle size of about 30 μm to obtain thepolytetrafluoroethylene molding powder of the present invention.

The polymerization conditions are shown in Table 1, and the physicalproperties of the obtained polyterafluoroethylene and the physicalproperties of the molded article made of this molding powder are shownin Table 2.

EXAMPLE 2 TO 7

The polymerization and pulverization are carried out in the same manneras in Example 1 under the polymerization conditions shown in Table 1 toobtain the polytetrafluoroethylene molding powders of the presentinvention, and the above various physical properties are measured. Theresults are shown in Table 2.

COMPARATIVE EXAMPLE 1 TO 8

The polymerization and pulverization are carried out in the same manneras in Example 1 under the polymerization conditions shown in Table 1 toobtain the polytetrafluoroethylene molding powders for comparison, andthe above various physical properties are measured. The results areshown in Table 2.

COMPARATIVE EXAMPLE 9 TO 10

The above various physical properties of commercially available TFM-1700(Farbwerke Hoechst Aktien Gesellschaft) and TG-70J (Du pont-MitsuiFluorochemicals Co., Ltd.) measured in the same manner in ComparativeExamples 1 to 8. The results are shown in Table 2.

                                      TABLE 1                                     __________________________________________________________________________    Polymerization                                                                       Example                                                                condition                                                                            1    2   3    4   5    6   7                                           __________________________________________________________________________    Temperature                                                                          50   50  45   50  50   50  50                                          (°C.)                                                                  Perfluoro                                                                            PPVE.sup.1)                                                                        PPVE                                                                              PPVE PPVE                                                                              PPVE.sup.2)                                                                        PPVE                                                                              PPVE                                        (vinyl ether)                                                                 Kind   85   85  85   85  280  85  55                                          Charge (g)                                                                    Initiator                                                                            APS.sup.3)                                                                         APS APS  APS APS  APS APS                                         Kind                                                                          Charge (mg)                                                                          715  2750                                                                              2750 715 715  715 715                                         Dispersing.sup.5)                                                                    --   --  --   0.4 --   4.0 --                                          agent (g)                                                                     Polymerization                                                                       16.6 9.2 14.0 13.0                                                                              21.0 11.5                                                                              10.5                                        time (hour)                                                                   __________________________________________________________________________    Polymerization                                                                       Comparative Example                                                    condition                                                                            1   2   3   4   5   6   7   8                                          __________________________________________________________________________    Temperature                                                                          70  70  70  50  40  70  70  50                                         (°C.)                                                                  Perfluoro                                                                            PPVE                                                                              PPVE                                                                              PPVE                                                                              PPVE                                                                              PPVE                                                                              PPVE                                                                              --  PPVE                                       (vinyl ether)                                                                 Kind   85  85  85  85  85  280     10                                         Charge (g)                                                                    Initiator                                                                            APS APS APS APS redox.sup.4)                                                                      APS APS APS                                        Kind                                                                          Charge (mg)                                                                          714 143 649 5500                                                                              1254/                                                                             143 143 715                                                               693                                                    Dispersing.sup.5)                                                                    --  --  0.4 --  --  --  --  --                                         agent (g)                                                                     Polymerization                                                                       3.8 15.7                                                                              5.9 5.9 13.6                                                                              5.1 4.6 8.5                                        time (hour)                                                                   __________________________________________________________________________     .sup.1) perfluoro (npropyl vinyl ether)                                       .sup.2) perfluoro (2propoxypropyl vinyl ether)                                .sup.3) ammonium persulfate                                                   .sup.4) APS/Na.sub.2 SO.sub.3                                                 .sup.5) ammonium perfluorooctanoate                                      

                                      TABLE 2                                     __________________________________________________________________________                          Example                                                           Character   1     2     3     4     5     6     7                   __________________________________________________________________________    Physical properties                                                                     Yield (kg)  12.3  12.4  12.3  12.4  12.4  12.4  12.3                of polymer                                                                              Perfluoro(vinyl ether)                                                                    0.062 0.070 0.075 0.106 0.172 0.090 0.04                          content (% by weight)                                                         Melt viscosity                                                                            5.1   4.7   5.0   0.82  5.3   0.60  7.1                           (poise × 10.sup.10)                                                     Specific surface area                                                                     1.5   --    --    6.0   --    8.0   2.5                           of powder (m.sup.2 /g)                                                        Heat of melting (J/g)                                                                     21.9  22.5  22.2  21.7  22.4  22.3  23.0                          Heat of crystallization                                                                   20.6  22.3  22.0  22.6  22.2  22.5  22.9                          (J/g)                                                               Physical properties                                                                     Flex life   1540  1140  1580  1230  1480  1550  1410                of sintered article                                                                     (10.sup.4 cycles)                                                             Creep resistance                                                              Load 140 kgf/cm.sup.2                                                         25° C. total deformation                                                           13.7  13.1  13.5  11.0  13.0  11.2  13.5                          (%)                                                                           25° C. permanent                                                                   4.5   4.5   4.6   4.4   4.7   4.3   4.8                           deformation (%)                                                               Load 70 kgf/cm.sup.2                                                          200° C. total deformation                                                          18.1  17.8  18.0  12.7  17.9  13.0  18.0                          200° C. permanent                                                                  6.2   5.3   5.7   4.2   6.0   4.5   7.0                           deformation (%)                                                               Tensile strength                                                                          411   420   405   370   405   365   458                           (kgf/cm.sup.2)                                                                Elongation (%)                                                                            408   415   410   366   410   370   405                           SVI         38    48    36    40    37    36    40                  __________________________________________________________________________                          Comparative Example                                               Character   1   2   3   4   5   6   7   8   9.sup.6)                                                                          10.sup.7)           __________________________________________________________________________    Physical properties                                                                     Yield (kg)  12.4                                                                              12.4                                                                              12.4                                                                              12.3                                                                              12.4                                                                              12.4                                                                              12.4                                                                              12.4                                                                              --  --                  of polymer                                                                              Perfluoro(vinyl ether)                                                                    0.075                                                                             0.075                                                                             0.063                                                                             0.080                                                                             0.071                                                                             0.160                                                                             None                                                                              0.005                                                                             0.12                                                                              0.053                         content (% by weight)                                                         Melt viscosity                                                                            2.3 4.0 0.51                                                                              4.0 2.1 3.9 15.9                                                                              9.0 2.3 1.1                           (poise × 10.sup.10)                                                     Specific surface area                                                                     2.7 --  5.0 --  --  --  --  3.0 2.7 3.3                           of powder (m.sup.2 /g)                                                        Heat of melting (J/g)                                                                     28.7                                                                              26.5                                                                              31.8                                                                              26.8                                                                              28.0                                                                              27.2                                                                              22.3                                                                              22.8                                                                              29.3                                                                              30.0                          Heat of crystallization                                                                   27.7                                                                              25.8                                                                              32.2                                                                              26.2                                                                              27.3                                                                              26.4                                                                              22.4                                                                              22.6                                                                              28.3                                                                              28.7                          (J/g)                                                               Physical properties                                                                     Flex life   350 620 270 580 420 530 750 720 360 180                 of molded article                                                                       (10.sup.4 cycles)                                                             Creep resistance                                                              Load 140 kgf/cm.sup.2                                                         25° C. total deformation                                                           11.0                                                                              11.9                                                                              11.2                                                                              11.8                                                                              11.7                                                                              14.8                                                                              16.7                                                                              16.0                                                                              11.0                                                                              9.7                           (%)                                                                           25° C. permanent                                                                   5.9 3.9 4.5 4.0 4.8 3.5 9.7 8.0 3.1 5.0                           deformation (%)                                                               Load 70 kgf/cm.sup.2                                                          200° C. total deformation                                                          14.6                                                                              17.2                                                                              17.1                                                                              16.8                                                                              15.4                                                                              16.6                                                                              30.6                                                                              28.0                                                                              14.2                                                                              17.1                          200° C. permanent                                                                  4.4 4.4 5.5 5.0 4.5 5.0 19.3                                                                              15.0                                                                              3.7 6.2                           deformation (%)                                                               Tensile strength                                                                          362 390 291 395 385 405 471 450 430 435                           (kgf/cm.sup.2)                                                                Elongation (%)                                                                            498 456 540 444 470 450 393 405 470 440                           SVI         62  53  65  55  60  58  270 210 70  76                  __________________________________________________________________________     .sup.6) TFM1700 (Farbwerke Hoechst Aktien Gesellschaft)                       .sup.7) TG70J (Du pontMitsui Fluorochemicals)                            

EXAMPLE 8

2 kg of the polytetrafluoroethylene molding powder produced in Example 1and 7 liters of water are charged in a stainless steel agglomerationvessel of 10 liters which a temperature inside the vessel is maintainedat 25° C. After they are stirred for 2 minutes with using oar typestirrer at 600 r.p.m., 770 ml of trifluorotrichloroethane is added andthe mixture is stirred for further 2 minutes. Then, the stirrer isreplaced by blade type stirrer having cracking ability, and the stirringis carried out for 5 minutes at 2000 r.p.m. Then, the blade type stirrerare replaced again by the oar type stirrer, and the mixture is heated to47° C. with stirring at 600 r.p.m. to vaporize and recover organicsolvents. After cooling down to 30° C., the product is taken out anddried for 24 hours at 120° C.

The above various physical properties of the resulting pelletized powderare measured. The results are shown in Table 3.

EXAMPLE 9

The agglomeration is carried out in the same manner as in Example 8except adding 730 ml of trifluorotrichloroethane.

The above physical properties of the resulting pelletized powder isshown in Table 3.

EXAMPLE 10

The agglomeration is carried out in the same manner as in Example 8except adding 780 ml of 1,3-dichloro-1,1,2,2,3-pentafluoropropane inplace of 770 ml of trifluorotrichloroethtane.

The above physical properties of the resulting pelletized powder isshown in Table 3.

COMPARATIVE EXAMPLE 11

The agglomeration is carried out in the same manner as in Example 8except using the polyterafluoroethylene molding powder for comparisonproduced in comparative example 1.

The above physical properties of the resulting pelletized powder forcomparison are shown in Table 3.

COMPARATIVE EXAMPLE 12

The agglomeration is carried out in the same manner as in Example 9except using the polytetrafluoroethylene molding powder for comparisonobtained in Comparative Example 1.

The above physical properties of the resulting pelletized powder forcomparison are shown in Table 3.

COMPARATIVE EXAMPLE 13

The agglomeration is carried out in the same manner as in Example 8except using the polytetrafluoroethylene molding powder for comparisonobtained in Comparative Example 7.

The above physical properties of the resulting pelletized powder forcomparison are shown in Table 3.

                                      TABLE 3                                     __________________________________________________________________________                    Example     Comparative Example                               Physical properties                                                                           8   9   10  11  12  13                                        __________________________________________________________________________    Average particle size (μm)                                                                 541 416 520 418 328 483                                       Apparent density (g/cm.sup.3)                                                                 0.82                                                                              0.84                                                                              0.81                                                                              0.79                                                                              0.81                                                                              0.75                                      Component 0.7 to 1.3 times larger                                                             52.1                                                                              43.8                                                                              50.5                                                                              44.9                                                                              39.0                                                                              51.2                                      than d.sub.50 (% by weight)                                                   Flex life (× 10.sup.4 cycles)                                                           1050                                                                              1230                                                                              1110                                                                              150 170 430                                       Welding factor  0.92                                                                              0.93                                                                              0.91                                                                              0.98                                                                              0.99                                                                              0.51                                      Number of pores (pore/m.sup.2)                                                                40  36  39  38  37  61                                        Creep resistance                                                              Load 140 kfg/cm.sup.2                                                         25° C. total deformation (%)                                                           13.9                                                                              13.8                                                                              13.8                                                                              11.5                                                                              11.2                                                                              17.1                                      25° C. permanent deformation (%)                                                       4.8 4.7 4.8 6.2 6.0 9.8                                       Load 70 kgf/cm.sup.2                                                          200° C. total deformation (%)                                                          18.5                                                                              18.3                                                                              18.4                                                                              14.8                                                                              14.8                                                                              30.7                                      200° C. permanent deformation (%)                                                      6.5 6.3 6.6 4.7 4.5 19.5                                      Tensile strength (kgf/cm.sup.2)                                                               403 405 402 355 360 392                                       Elongation (%)  401 405 406 485 495 351                                       SVI             39  35  39  76  60  278                                       __________________________________________________________________________

COMPARATIVE EXAMPLE 11

A specific PTFE molding powder having a low heat of crystallizationproduced in Example 1 and carbon fibers (diameter 7 μm, average fiberlength 70 μm) are drymixed in a ratio by weight of 90/10 to obtain themolding composition of the present invention. This molding compositionis molded by a compression molding method (final pressure is 500 kgf/cm²and maintained for 5 minutes), then sintered to produce a cylindricalmolded article. An abrasion resistance (an abrasion coefficient, afriction coefficient), a creep resistance, a tensile strength and anelongation of this molded article are measured. The results are shown inTable 4.

EXAMPLE 12

The PTFE molding powder having a low heat of crystallization produced inExample 1 and glass fibers (diameter 11 μm, average fiber length 40 μm)are dry-mixed in a ratio by weight of 90/10 to obtain a moldingcomposition. This composition is molded and sintered in the same manneras in Example 11, and physical properties of the sintered article aremeasured. The results are shown in Table 4.

EXAMPLE 13

The PTFE molding powder having a low heat of crystallization produced inExample 1, carbon fibers (diameter 7 μm, average fiber length 70 μm) andpolyoxybenzoylpolyester (Econol(®) E-101S, sumitomo Chemical Co., Ltd.)are dry-mixed in a ratio by weight of 80/5/15 to obtain a moldingcomposition. This composition is molded and sintered in the same manneras in Example 11, and the above physical properties of the sinteredarticle are measured. The results are shown in Table 4.

COMPARATIVE EXAMPLE 14

The PTFE molding powder produced in Comparative Example 1 and carbonfibers are mixed in a ratio by weight of 90/10 in the same manner as inExample 11 to obtain a molding composition. This composition is moldedand sintered in the same manner as in Example 11, and the above physicalproperties of the sintered article are measured. The results are shownin Table 4.

COMPARATIVE EXAMPLE 15

PTFE molding powder produced in Comparative Example 1 and glass fibersare mixed in a ratio by weight of 90/10 in the same manner as in Example12 to obtain a molding composition. This composition is molded andsintered in the same manner as in Example 11, and the above physicalproperties of the sintered article are measured. The results are shownin Table 4.

COMPARATIVE EXAMPLE 16

The PTFE molding powder produced in Comparative Example 7 and carbonfibers are mixed in a ratio by weight of 90/10 in the same manner as inExample 11 to obtain a molding composition. This composition is moldedand sintered in the same manner as in Example 11, and the above physicalproperties of the sintered article are measured. The results are shownin Table 4.

COMPARATIVE EXAMPLE 17

The PTFE molding powder produced in Comparative Example 7 and glassfibers are mixed in a ratio by weight of 90/10 in the same manner inExample 12 to obtain a molding composition. This composition is moldedand sintered in the same manner as in Example 11, and the above physicalproperties of the sintered article are measured. The results are shownin Table 4.

                                      TABLE 4                                     __________________________________________________________________________                          Example     Comparative Example                                  Character    11 12   13  14  15  16  17                              __________________________________________________________________________    Physical properties                                                                    Yield (kg)   12.3                                                                              12.3                                                                              12.3                                                                              12.4                                                                              12.4                                                                              12.4                                                                              12.4                            of polymer                                                                             Perfluoro(vinyl ether)                                                                     0.062                                                                             0.062                                                                             0.062                                                                             0.075                                                                             0.075                                                                             None                                                                              None                                     content (% by weight)                                                         Melt viscosity                                                                             5.1 5.1 5.1 2.3 2.3 15.9                                                                              15.9                                     (poise × 10.sup.10)                                                     Specific surface area                                                                      1.5 1.5 1.5 2.7 2.7 1.7 1.7                                      of powder (m.sup.2 /g)                                                        Heat of melting (J/g)                                                                      21.9                                                                              21.9                                                                              21.9                                                                              28.7                                                                              28.7                                                                              22.2                                                                              22.2                                     Heat of crystallization                                                                    20.6                                                                              20.6                                                                              20.6                                                                              27.7                                                                              27.7                                                                              22.4                                                                              22.4                                     (J/g)                                                                Physical properties                                                                    Adhesion resistance                                                                        2.3 4.5 1.5 5.5 10.5                                                                              7.4 14.5                            of molded article                                                                      Abrasion coefficient.sup.1)                                                   (× 10.sup.-5)                                                           Friction coefficient                                                                       0.22                                                                              0.24                                                                              0.23                                                                              0.30                                                                              0.26                                                                              0.14                                                                              0.20                                     Creep resistance                                                              Load 140 kgf/cm.sup.2                                                         25° C. total deformation (%)                                                        6.2 7.1 4.6 5.9 6.9 9.4 12.4                                     25° C. permanent                                                                    2.2 2.6 2.5 1.9 2.4 3.4 6.7                                      deformation (%)                                                               Load 70 kgf/cm.sup.2                                                          200° C. total deformation                                                           7.4 12.0                                                                              6.1 7.0 11.7                                                                              10.6                                                                              17.0                                     200° C. permanent                                                                   2.5 4.8 3.1 2.1 4.5 3.7 7.3                                      deformation (%)                                                               Tensile strength                                                                           227 191 186 241 200 304 270                                      (kgf/cm.sup.2)                                                                Elongation (%)                                                                             292 327 324 350 480 293 325                             __________________________________________________________________________     .sup.1) Unit: mm/km/kgf/cm.sup.2                                         

EXAMPLE 14

A stainless steel agglomeration vessel of 10 liters is charged with 2 kgof the molding composition containing carbon fibers produced in Example11 and 7 liters of water, a temperature inside the vessel beingmaintained at 25° C. After they are stirred for 2 minutes with using oartype stirrer at 600 r.p.m, 700 ml of trifluorotrichloroethane is addedand the mixture is stirred for further 2 minutes. Then, the stirrer isreplaced by blade type stirrer having cracking ability, and stirring iscarried out for 5 minutes at 2000 r.p.m. Then, the blade type stirrerare replaced again by the oar type stirrer, and the mixture is heated to47° C. with stirring at 600 r.p.m. to vaporize and recover organicsolvents. After cooling down to 30° C., the product is taken out anddried for 24 hours at 120° C.

The above various physical properties of the resulting moldingpelletized powder containing carbon fibers are measured. The results areshown in Table 5.

EXAMPLE 15

The agglomeration is carried out in the same manner as in Example 14with using 2 kg of the molding composition containing glass fibersproduced in Example 12. The above various physical properties of theresulting molding pelletized powder containing glass fibers aremeasured. The results are shown in Table 5.

EXAMPLE 16

The molding composition produced in Example 13 is pelletized in the samemanner as in Example 14. The above various physical properties of theresulting molding pelletized powder are measured. The results are shownin Table 5.

EXAMPLE 17

The agglomeration is carried out in the same manner as in Example 14except using 2 kg of the molding composition containing carbon fibersproduced in Example 11 and using as a solvent 720 ml of1,3-dichloro-1,1, 2,2, 3-pentafluoropropane. The above various physicalproperties of the resulting pelletized powder containing carbon fibersare measured. The results are shown in Table 5.

EXAMPLE 18

The agglomeration is carried out in the same manner as in Example 14except using 2 kg of the molding composition containing glass fibersproduced in Example 12 and using as a solvent 720 ml of 1, 1-dichloro-2,2,3,3, 3-pentafluoropropane. The above various physicalproperties of the resulting pelletized powder containing glass fibersare measured. The results are shown in Table 5.

COMPARATIVE EXAMPLE 18

The PTFE molding composition produced in Comparative Example 14 ispelletized in the same manner as in Example 14. The above variousphysical properties of the resulting pelletized powder containing carbonfibers are measured. The results are shown in Table 5.

COMPARATIVE EXAMPLE 19

The PTFE molding composition produced in Comparative Example 15 ispelletized in the same manner in Example 14. The above various physicalproperties of the resulting pelletized powder containing glass fibersare measured. The resluts are shown in Table 5.

COMPARATIVE EXAMPLE 20

The PTFE molding composition produced in Comparative Example 16 ispelletized in the same manner as in Example 14. The above variousphysical properties of the resulting pelletized powder containing carbonfibers are measured. The resluts are shown in Table 5.

COMPARATIVE EXAMPLE 21

The PTFE molding composition produced in Comparative Example 17 ispelletized in the same manner as in Example 14. The above variousphysical properties of the resulting pelletized powder containing glassfibers are measured. The resluts are shown in Table 5.

                                      TABLE 5                                     __________________________________________________________________________                 Example         Comparative Example                              Physical properties                                                                        14  14  16  17  18  18  19  20  21                               __________________________________________________________________________    Average particle size (μm)                                                              520 481 392 505 492 405 385 462 450                              Apparent density                                                                           0.75                                                                              0.77                                                                              0.71                                                                              0.73                                                                              0.76                                                                              0.73                                                                              0.77                                                                              0.74                                                                              0.73                             Component 0.7 to 1.3 times                                                                 53.5                                                                              43.5                                                                              87.4                                                                              56.1                                                                              57.2                                                                              46.2                                                                              40.5                                                                              48.5                                                                              47.3                             larger than d.sub.50 (% by weight)                                            Welding factor                                                                             0.75                                                                              0.81                                                                              0.53                                                                              0.76                                                                              0.80                                                                              0.85                                                                              0.90                                                                              0.64                                                                              0.71                             Number of pores (pore/m.sup.2)                                                             105 .sup.1)                                                                           152 102 .sup.1)                                                                           98  .sup.1)                                                                           122 .sup.1)                          Abrasion resistance                                                                        2.8 4.7 1.4 2.7 4.7 5.6 10.6                                                                              7.5 14.7                             Abrasion coefficient (× 10.sup.-5)                                      Friction coefficient                                                                       0.25                                                                              0.26                                                                              0.23                                                                              0.26                                                                              0.25                                                                              0.31                                                                              0.28                                                                              0.15                                                                              0.21                             Creep resistance                                                              Load 140 kgf/cm.sup.2                                                         25° C. total deformation (%)                                                        6.5 7.3 4.6 6.4 7.2 6.0 7.1 9.5 12.5                             25° C. permanent                                                                    2.3 2.8 2.6 2.2 2.8 2.1 2.6 3.4 6.7                              deformation (%)                                                               Load 70 kgf/cm.sup.2                                                          200° C. total deformation (%)                                                       7.7 12.2                                                                              6.3 7.5 12.3                                                                              7.1 11.7                                                                              10.8                                                                              17.2                             200° C. permanent                                                                   2.8 5.0 3.1 2.7 5.0 2.1 4.7 3.7 7.4                              deformation (%)                                                               Tensile strength (kgf/cm.sup.2)                                                            205 180 140 212 192 215 175 265 233                              Elongation (%)                                                                             245 282 277 260 280 312 405 252 282                              __________________________________________________________________________     .sup.1) Innumerable number of pores are supposed to exist since current       continuosly flowed during the measurement                                

EXAMPLE 19

A mold having an internal diameter of 50 mm is charged with 320 g of thepowder produced in the Production Example 1. The powder is graduallypressurized for about 30 seconds to a final pressure of 300 Jg/cm², andthe final pressure is further maintained for 5 minutes to produce apreform. The preform is taken out of the mold, and heated at a rate of50° C./h to 365° C. in an air oven. After maintaining the temperaturefor 5.5 hours, the sintered article is cooled down to room temperatureat a rate of 50° C./h to produce a cylindrical sintered article. Thissintered article is machined to obtain the molded article (bellows)Shown in FIG. 2. This molded article is installed to a bellows-pump(2KBM-1ZU1, IWAKI CO. LTD), and a water-operation (32 r.p.m., dischargerate 75 ml/min) is carried out at 25° C. for 720 hours. Then, thebellows is detached from the pump, and bellows part is observed witheyes.

The results are shown in Table 6.

EXAMPLES 20 to 22 AND COMPARATIVE EXAMPLES 22 TO 24

A molded article is produced in the same manner in Example 19 exceptusing molding powders shown in Table 6, the water-operation is carriedcut by bellows-pump, and bellows part is observed with eyes. The reslutsare shown in Table 6.

                  TABLE 6                                                         ______________________________________                                                                   Bellows condition                                  Example      Molding powder                                                                              after operation                                    ______________________________________                                        Example 19   Example 1     No change                                          Example 20   Example 3     Slightly whity                                     Example 21   Example 5     No change                                          Example 22   Example 8     No change                                          Comparative  Comparative   Extremely whity                                    Example 22   Example 1                                                        Comparative  Comparative   Whity                                              Example 23   Example 7                                                        Comparative  Comparative   Extremely whity                                    Example 24   Example 11                                                       ______________________________________                                    

The polytetrafluoroethylene molding powder of the present inventionprovides molded articles which are excellent both in the creepresistance and the flex fatigue resistance (the flex life), and isextremely effective as a molding material for parts which are repeatedlybent such as bellows, diaphragm, pump and the like.

INDUSTRIAL APPLICABILITY

Because of its excellent characteristics, the molding powder accordingto the present invention is useful as it is or after blending with thefiller, as a raw material for molded articles for the following uses.For example, molded articles which need a flex resistance such asbellows, disphragm, hose, piston ring and butterfly valve; moldedarticles which need a creep resistance such as ball valve sheet,packing, gasket, piston ring, bellows, diaphragm and butterfly valve;molded articles which need a gas permeation resistance such as bellows,disphragm, hose, packing and gasket; and the like.

We claim:
 1. A polytetrafluoroethylene molding powder which is notmelt-processable and which has a specific surface area of 0.5 to 0.9 m²/g and an average particle size of not more than 100 μm, characterizedin that the polytetrafluoroethylene comprises 0.01 to 1% by weight of aperfluoro(vinyl ether) unit having the general formula (I): ##STR2##wherein X is a perfluoroalkyl group having 1 to 6 carbon atoms or aperfluoroalkoxyalkyl group having 4 to 9 carbon atoms, and has a heat ofcrystallization of 18.0 to 25.0 J/g measured by a differential scanningcalorimeter, and that a molded article made of the powder has(a) a flexlife of not less than 7 million cycles, (b) a creep resistance (totaldeformation) of not more than 20% at 200° C. measured after a retentionfor 24 hours of a load of 70 kgf/cm² and (c) a creep resistance (totaldeformation) of not more than 15% at 25° C. measured after a retentionfor 24 hours of a load of 140 kgf/cm².
 2. The polytetrafluoroethylenemolding powder of claim 1, wherein the polytetrafluoroethylene comprises0.03 to 0.20% by weight of the perfluoro(vinyl ether) unit.
 3. Thepolytetrafluoroethylene molding powder of claim 1, wherein thepolytetrafluoroethylene has a melt viscosity (380° C.) of 5.0×10⁹ to1×10¹¹ poises.
 4. The polytetrafluoroethylene molding powder of claim 1,wherein the polytetrafluoroethylene has a heat of crystallization of18.0 to 23.5 J/g and the molded article has a flex life of not less than10 million cycles.
 5. The polytetrafluoroethylene molding powder ofclaim 1, wherein the polytetrafluoroethylene has a SVI of not more than50.
 6. The polytetrafluoroethylene molding powder of claim 1, whereinthe polytetrafluoroethylene has a specific surface area of 5.0 to 9.0 m²/g.
 7. The polytetrafluoroethylene molding powder of claim 1, whereinthe perfluoro(vinyl ether) unit is perfluoro(propyl vinyl ether) unit.8. The polytetrafluoroethylene molding powder of claim 1, wherein theperfluoro(vinyl ether) unit is perfluoro(2-propoxypropyl vinyl ether)unit.
 9. A process for preparing the polytetrafluoroethylene moldingpowder according to claim 1, characterized in that tetrafluoroethyleneand a perfluorovinyl ether having the general formula (II):

    CF.sub.2 ═CF--O--X                                     (II)

wherein X is a perfluoroalkyl group having 1 to 6 carbon atoms or aperfluoroalkoxyalkyl group having 4 to 9 carbon atoms, aresuspension-polymerized at a temperature of 40° to 55° C. by using apersulfate as an initiator in the substantial absence of dispersingagent.
 10. The process for preparing the polytetrafluoroethylene moldingpowder of claim 9, wherein the suspension polymerization is carried outby adding an dispersing agent which is telogenically inactive in anamount of 1 to 200 ppm on the basis of polymerization water.
 11. Theprocess of claim 9 in which the perfluoro(vinyl ether) isperfluoro(propyl vinyl ether).
 12. The process of claim 9, wherein theperfluoro(vinyl ether) is perfluoro(2-propoxypropyl vinyl ether). 13.The process of claim 9, wherein a persulfate having a half life at 55°C. of 18 to 120 hours is used, and, at the beginning of thepolymerization, the initiator is fed in such an amount that adecomposition amount of the initiator during three hours from thebeginning of the polymerization at a polymerization temperature of 40°to 55° C. is 4×10⁻⁷ to 8×10⁻⁶ mole/liter in terms of a concentration onthe basis of the polymerization water.
 14. A filler-containingpolyterafluoroethylene molding composition comprising a mixture of thepolyterafluoroethylene molding powder of claim 1, and a filler in aratio by weight of 99 to 20/80 to
 1. 15. The molding composition ofclaim 14, wherein the filler is glass fiber, graphite powder, bronzepowder, gold powder, silver powder, copper powder, stainless steelpowder, stainless steel fiber, nickel powder, nickel fiber, molybdenumdisulfide powder, carbon black, cokes powder, carbon fiber, aromaticheat resistant resin powder, boron nitride powder, polyimide powder, PFApowder, fluorinated mica powder, polyphenylene sulfide powder or mixturethereof.
 16. A polytetrafluoroethylene molded article obtainable bymolding the polytetrafluoroethylene molding powder of claim 1,characterized in that the polytetrafluoroethylene molded article has(a)a flex life of not less than 7 million cycles, (b) a creep resistance(total deformation) of not more than 20% at 200° C. and (c) a creepresistance (total deformation) of not more than 15% at 25° C.
 17. Themolded article of claim 16, wherein the polytetrafluoroethylene has aheat of crystallization of 18.0 to 23.5 J/g and the molded article has aflex life of not less than 10 million cycles.
 18. The molded article ofclaim 16, wherein the molded article is a molded article for sealing.19. The molded article of claim 16, wherein the molded article is aflexible molded article.
 20. A polytetrafluoroethylene molded articleobtainable by molding a pelletized powder of claim 14, characterized inthat the polytetrafluoroethylene molded article has(a) a flex life ofnot less than 5 million cycles, (b) a creep resistance (totaldeformation) of not more than 20% at 200° C. and (c) a creep resistance(total deformation) of not more than 15% at 25° C.
 21. A process forpreparing the polytetrafluoroethylene molding powder according to claim1, characterized in that tetrafluoethylene and a perfluorovinyl etherhaving the general formula (II):

    CF.sub.2 ═CR--O--X                                     (II)

wherein X is a perfluoroalkyl group having 1 to 6 carbon atoms or aperfluoroalkoxyalkyl group having 4 to 9 carbon atoms, aresuspension-polymerized at a temperature of 40° to 55° C. by using apersulfate as an initiator and by adding an dispersing agent which istelogenically inactive in an amount of 1 to 200 ppm on the basis ofpolymerization water.
 22. A process for preparing thepolytetrafluoroethylene molding powder according to claim 1,characterized in that tetrafluoroethylene and a perfluorovinyl etherhaving the general formula (II):

    CF.sub.2 ═CR--O--X                                     (II)

wherein X is a perfluoroalkyl group having 1 to 6 carbon atoms or aperfluoroalkoxy alkyl group having 4 to 9 carbon atoms, aresuspension-polymerized at a temperature of 40° to 55° C. by using apersulfate as an initiator having a half life at 55° C. of 18 to 120hours and by adding an dispersing agent which is telogenically inactivein an amount of 1 to 200 ppm on the basis of polymerization water and,at the beginning of the polymerization, the initiator is fed in such anamount that a decomposition amount of the initiator during three hoursfrom the beginning of the polymerization at a polymerization temperatureof 40° to 55° C. is 4×10⁻⁷ to 8×10⁻⁶ mole/liter in terms of aconcentration on the basis of the polymerization water.
 23. Apolyterafluoroethylene molding powder which is not melt-processable andwhich has a specific surface area of 0.5 to 9.0 m² /g and an averageparticle size of not more than 100 μm, characterized in that thepolytetrafluoroethylene comprising 0.01 to 1% by weight of aperfluoro(vinyl ether) unit having the general formula (I): ##STR3##wherein X is a perfluoroalkyl group having 1 to 6 carbon atoms or aperfluoroalkoxyalkyl group having 4 to 9 carbon atoms, and has a heat ofcrystallization of 18.0 to 25.0 J/g measured by a differential scanningcalorimeter,wherein the molding powder is prepared bysuspension-polymerizing tetrafluoroethylene and the perfluoro(vinylether) at a temperature of 40° to 55° C., and a molded article made ofthe powder has(a) a flex life of not less than 7 million cycles, (b) acreep resistance (total deformation) of not more than 20% at 200° C.measured after a retention of 24 hours of a load of 70 kgf/cm² and (c) acreep resistance (total deformation) of not more than 15% at 25° C.measured after a retention for 24 hours of a load of 140 kgf/cm².